The discharge of heavy metals in different wastewaters contaminates the surface water and disturbs the biological purification processes in municipal sewage plants. The dominating purification technique currently used is chemical precipitation of the metals as hydroxides. Hydroxide sludge is obtained as a final product, which results in transport and deposition costs. Metal ions have been replaced by sodium or calcium ions in the wastewater, which further means that this salt-bearing water cannot be returned to the process.
The ideal method of purifying wastewaters containing heavy metals would be a precipitation of the metals in a pure form with simultaneous desalination of the water to a purity allowing recycling in the process. The present invention enables an electrolytic metal precipitation which gives these advantages. Electrolytic metal precipitation from diluted solutions has previously not come into industrial use, due to the fact that conventional electrolysis with planar electrodes requires too large electrode areas to give a satisfactory purification effect, with the low final contents which are required these days (approximately 1 ppm). Under these conditions the electrolysis process is controlled by the transport of metal ions to the electrode surface (the cathode). To achieve a high material transport rate per volumetric unit, and thereby a small reactor volume, the cathode can instead be formed in accordance with the invention as a percolated porous body or a stable particulate bed with a high specific area.
The basis principle of using beds of conductive grains as electrodes to obtain high specific areas is indeed described in the literature. In this connection reference is made to e.g. DOS No. 2,022,497, DOS No. 2,904,539, U.S. Pat. Nos. 3,974,049, 3,859,195, 3,459,646, 3,647,653 and 3,650,925 and DE Nos. 2,424,001, 2,620,702, 2,705,007 and 2,723,708 and U.S. Pat. Nos. 4,123,340 and 4,217,191 and SE No. 413,446. However, none of these more or less sophisticated structures should be particularly suitable for industrial application with the severe requirements now made on wastewaters, and the structures are furthermore so fixed in their constructions that they cannot be adapted or modified simply and easily to different applications or conditions as is the case with the electrodes in accordance with the present invention.
Cell structures of the filter press type are indeed also known, reference e.g. being made to U.S. Pat. No. 4,274,939, but in these cases the electrodes are of a completely different structure, viz. in the form of solid plates, which structure cannot possibly be compared with the beds of conductive grains which have so far been used within the field of porous percolation electrodes. Other examples of frames with solid electrodes are those disclosed in DE No. 3,221,371 A1 and SU No. 619,551.
Thus, the art of using a bed of conductive grains as a percolation electrode must be considered specifically distinct from the art of using planar electrodes. Frames with planar electrodes have been known for a long time but as far as we know nobody has ever now the idea of utilizing percolation beds of distinct grains in frames or even less solved the problem of accomplishing this in a useful way. The present invention relates to an electrode chamber unit which solves the specific and different problems associated with a particulate electrode having good purification capacity. For instance, such a structure is extremely sensitive to so called channel-formation. Apart from the fact that the unit according to the invention does not comprise outer and inner frames as in U.S. Pat. No. 4,274,939, the grid structure and scaling properties of the frames used in said U.S. patent would not enable the provision of a unit in accordance with the present invention. The extremely high degrees of purification that have been obtained (up to 99.99%; cf. the Examples) represent a major advantage of the unit according to the invention, especially in the light of the great versatility of the invention as compared to previously known beds of conducting grains, and are not obtainable by the known frames.
The present invention has thus been found particularly usable in conjunction with the purification of wastewaters, e.g. washing water from the galvanizing industry, or mine water, but the new electrode chamber units and cells are by no means limited to just this use, but one of the great advantages of the invention is just that electrochemical cells can be tailored for practically any kind of electrochemical reaction of the electrolysis type.
A flexible, universally usable cell must meet other demands than those met by cells having beds of conductive grains. In common there is the demand on small electrode distances and the desirability of some form of package principle. However, the electrodes must be easily exchangeable, since different processes require different electrode materials. Furthermore, the cell must also be designed in a material that withstands corrosion in as many conceivable electrolytes as possible. It is also known that many metals disturb the electrode processes and cause poisoning of the electrodes. It would therefore be desirable with a cell in an inert plastics material. If the components of the cell can be injection moulded, the precision required for proper sealing can be achieved. Furthermore the price can be kept low if the series are reasonably long. Injection moulding requires, however, that the number of differently shaped parts can be kept down.
All this is enabled by the new electrode chamber unit in accordance with the present invention.